Local and regional restrictions to tackle the Indian Covid variant cannot be ruled out, the government has said, as it expressed concern over the speed of the variant's spread in England.
It said "economic and social" restrictions could be reimposed in parts of England if necessary.
Measures could also include bringing forward second vaccines for some people in the worst affected areas.
Public Health England has recorded 1,313 UK cases of the Indian variant.
The figures released on Thursday are more than double the 520 cases recorded by PHE up to 5 May.
The Department of Health and Social Care (DHSC) said there was "no firm evidence yet to show this variant has any greater impact on severity of disease or evades the vaccine".
But it also said ministers "cannot rule out reimposing economic and social restrictions at a local or regional level if evidence suggests they are necessary to contain or suppress a variant which escapes the vaccine".
Health Secretary Matt Hancock said the government was supporting areas where cases were rising and urged people to "continue to be vigilant".
"We are monitoring the situation very carefully and will not hesitate to take further action if necessary," he said.
Earlier, Prime Minister Boris Johnson did not rule out reintroducing regional restrictions, saying: "It is a variant of concern, we are anxious about it."
But he added there was no evidence to suggest England's planned lockdown easing could not go ahead.
The next major easing of restrictions is due to take place on 17 May.
On Thursday, the UK reported another 11 deaths within 28 days of a positive test and a further 2,657 coronavirus cases.
Almost 36 million people have also received at least one dose of a coronavirus vaccine and on Friday Public Health England said vaccines had saved 11,700 lives and stopped 33,000 people becoming seriously ill with the disease in England.
Surge testing is already taking place in 15 areas across England. This is when increased testing and enhanced contact tracing is carried out in specific locations to try to prevent the spread of outbreaks.
The DHSC said additional measures where "clusters" of cases are detected would include increased genome sequencing of cases; increased community engagement; and encouraging uptake of the vaccine for those who are eligible.
It said ministers were also considering bringing forward second doses for people who are eligible.
Cases of the Indian variant in the UK have been recorded in London, Bolton, Tyneside and Nottingham.
PHE said mobile testing units had been set up in Bolton, and door-to-door PCR testing had been offered to 22,000 residents. A PCR test is the most accurate way to check for Covid.
A vaccine bus has been set up in the town to increase uptake among those who are eligible for vaccinations and a rapid response team of 100 nurses, public health advisers and environmental health officers has been sent in.
Extra clinics will open in Blackburn and Darwen in Lancashire from next week to offer the vaccine to those who are eligible under national guidelines.
Surge testing has also been deployed in Sefton, Merseyside, after cases of the Indian variant were confirmed in the Formby area, with anyone over 16 who lives, works or studies in the area urged to take a PCR test.
The India variant - officially called B.1.617.2 - is one of four mutated versions of coronavirus which have been designated as being "of concern" by UK public health bodies, with others first being identified in Kent, South Africa and Brazil.
Experts believe the Indian variant has mutations that may make it more transmissible, but it is not yet known if it causes more severe disease or if it might make current vaccines less effective.
Prof Robert Dingwall, a scientist on the government's New and Emerging Respiratory Virus Threats Advisory Group (Nervtag), said it seemed people who had been vaccinated "have only a very low risk of infection" from the Indian variant - and infection was "likely to be mild".
But sources have told the BBC surge testing in areas where the variant has been found "isn't working", with the current strategy identifying cases but no stopping the spread.
The sources added cases of the variant were being seen in many places with no links to travel and case numbers had been "grossly underestimated".
Greater Manchester mayor Andy Burnham told the BBC's Newscast podcast he would be "very concerned" about the possibility of local restrictions being imposed in the region while the rest of the country was unlocking.
Speaking following the prime minister's comments earlier in the day, the Labour mayor said he would like to see younger people in affected areas offered vaccines, including extending jabs to 16 to 18-year-olds.
Questions are also being raised about the potential need to delay stage four, due on 21 June, when the government hoped to remove all legal limits on social contact.
Senior Tory Steve Baker, deputy chairman of the Covid Recovery Group of MPs, warned against any further lockdown measures, asking: "Why on earth would we lock down when the vaccines continue to break the link between cases and hospitalisations and deaths?"
BBC
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